Erythrocytes infected with malarial parasites are characteristically phagocytized in vivo in infected animals (and man). The intracellular processing of the parasite material by cells of the reticuloendothelial system (RES) may be instrumental in the induction of immunity to malaria. However, before such immunity becomes functional, parasitemia may overwhelm the host. Using the rodent malarial parasites Plasmodium vinckei chabaudi and P. berghei, we are attempting, by use of explanted cells of the RES, to process infected erythrocytes in vitro. This will render the parasites within them not only non-infectious but also capable of serving, following infection into suitable laboratory animals, as an antigen adequate to stimulate specific anti-malarial antibody. This anti- malarial antibody may be assessed by direct malarial challenge or by in vitro methods.